LIM transportation systems are well-known. The most common type involves a vehicle-borne primary which generates the moving magnetic field required to propel the vehicle and a track-side secondary which is simply a reaction rail with a steel backing member and an aluminum facing. The steel backing member together with the primary define a magnetic path for the magnetic field generated by the primary, and the aluminum facing provides a low resistance medium in which secondary currents necessary for generation of thrust are induced. In some applications, the reaction rail may be constructed as a ladder-like structure with transverse conductor bars of varying resistance to alter the thrust characteristics of the system. In theme-park and airport systems, the arrangement of the LIM primary and secondary are often reversed: a track-side primary is provided, and the vehicle carries the secondary, an aluminum-over-steel reaction plate, marginally spaced from the primary.
The present invention addresses the problem of providing power for the electrical system of a LIM vehicle which carries a LIM secondary. An advantage commonly touted for such systems is that no exposed track-side power rail is required. However, to supply auxiliary power for operation of lights and fans, etc, it is common, in known or proposed systems to provide track-side rails and contact shoes carried by the vehicle which slip over the surface of the power rails. This of course introduces the usual problems of sliding contacts which the track-based LIM primary is intended to eliminate.
Further, in induction motors, both linear and rotary, a portion of the power crossing the air gap from the primary to the secondary is not converted into mechanical power. Such power is called "slip power" and in simple, conventional motors it is dissipated as heat in the secondary. At high slip, the efficiency of an induction motor can be low, with a correspondingly high slip power loss. This is particularly important for LIM powered vehicles which may operate at high slips, for example when stopping and starting.
It has been proposed to recover slip power from the rotor or secondary of conventional induction motors and then to invert the power and feed it back into the supply line. Alternatively, the recovered power can be fed to supplementary stator coils. However, this proposal is for a rotary motor and requires the provision of slip rings, to transfer the recovered power out from the rotor.
The present invention is directed to the problem of reducing slip power losses and to powering the auxiliary systems of a LIM vehicle, preferably without requiring sliding connections.